Field of the Invention
The present invention relates to a Raman amplification pumping module which supplies pumping light for Raman amplification of signal light, a Raman amplifier including the Raman amplification pumping module, and an optical communication system including the Raman amplifier.
Related Background Art
The Raman amplifier is an optical component which supplies Raman amplification pumping light into an optical fiber being an optical waveguide, thereby Raman-amplifying signal light propagating in the optical fiber. For example, the wavelength of the signal light is in the 1.55xcexcm band and the wavelength of the pumping light is near 1.45 xcexcm. For the Raman amplification of signal lightwaves of multiple wavelength channels, an important point is that the gain of Raman amplification is flat in a signal wavelength band including these signal channels. A conceivable means for flattening the gain is use of a gain equalizer having a loss spectrum of a shape similar to a spectrum shape of the Raman amplification gain. Another conceivable method is use of the Raman amplification pumping light with a plurality of peak wavelengths. The latter is rather preferable in that the signal light suffers no loss and the energy of the pumping light can be effectively used.
For example, the Raman amplifiers described in Document xe2x80x9cY. Emori, et al., xe2x80x9c100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodes,xe2x80x9d OFC ""99, PD19 (1999)xe2x80x9d use as the Raman amplification pumping light, multiplexed lightwaves resulting from multiplexing of lightwaves of twelve wavelength channels different from each other, in order to flatten the gain of Raman amplification in the signal wavelength band with the bandwidth of 100 nm. The light of each signal channel is obtained by polarization multiplexing of lightwaves in mutually orthogonal polarization states emitted from two semiconductor laser sources. In this Document, wavelength spacings between the 12-channel lightwaves are 7.5 nm or 15 nm. The Raman amplifiers disclosed in Japanese Patent Application Laid-Open No. 2000-98433 use as the pumping light the multiplexed lightwaves obtained in such a way that, in view of efficiency and gain flattening in multiplexing of lightwaves of multiple channels, the intervals between center wavelengths of lightwaves emitted from respective light sources are set in the range of not less than 6 nm nor more than 35 nm and these lightwaves are multiplexed to yield the multiplexed lightwaves.
The inventors investigated the foregoing prior art and found the following problem as a result of the investigation.
Namely, the aforementioned prior art had the problem that with a breakdown in either one of the light sources, the flatness of the Raman amplification gain was impaired to degrade the transmission quality of the signal light and in the polarization multiplexing case the transmission quality of the signal light was degraded furthermore.
The present invention has been accomplished in order to solve the above problem and an object of the present invention is to provide a Raman amplification pumping module having a configuration for effectively suppressing the degradation of the quality of the signal light even with a breakdown in either one of light sources, a Raman amplifier including the Raman amplification pumping module, and an optical communication system including the Raman amplifier.
A Raman amplification pumping module according to the present invention is configured to supply pumping light for Raman amplification of signal light propagating in an optical waveguide, into the optical waveguide. The Raman amplification pumping module comprises a pumping light generating system including a plurality of light sources; and a multiplexer for multiplexing a plurality of lightwaves outputted from the pumping light generating system. The plurality of light sources in the pumping light generating system emit lightwaves with their respective center wavelengths different from each other (differences between which are greater than 0 nm).
Particularly, in the case where, in the Raman amplification pumping module according to the present invention, lightwaves emitted from the plurality of light sources are subjected to polarization multiplexing, a difference between two center wavelengths selected out of the center wavelengths of those lightwaves is less than 6 nm. In the case where the lightwaves emitted from the plurality of light sources are not subjected to polarization multiplexing, a difference between two center wavelengths selected out of the center wavelengths of the lightwaves is practically 1 nm or more, preferably 2 nm or more.
Specifically, where one or more light source pairs are composed of light sources with center wavelengths of output lightwaves adjacent to each other, selected out of the plurality of light sources, wavelength spacings between the center wavelengths of the respective lightwaves emitted from the light sources included in these light source pairs are set less than 6 nm in the case where the lightwaves emitted from the plurality of light sources are subjected to polarization multiplexing. On the other hand, in the case where the lightwaves emitted from the plurality of light sources are not subjected to polarization multiplexing, a wavelength spacing between center wavelengths of lightwaves emitted from respective light sources included in one light source pair is practically set 1 nm or more, preferably 2 nm or more. Conversely, when attention is focused on the relation between the light source pairs, as to mutually different light source pairs including light sources which emit lightwaves with center wavelengths adjacent to each other, a wavelength spacing may be not less than 6 nm between a center wavelength of a lightwave emitted from one light source and a center wavelength of a lightwave emitted from another light source.
A Raman amplifier according to the present invention comprises an optical waveguide in which signal light propagates and which Raman-amplifies the signal light with supply of pumping light; and a Raman amplification pumping module of structure as described above, for supplying the pumping light into the optical waveguide (which is the Raman amplification pumping module according to the present invention). Further, an optical communication system according to the present invention comprises a Raman amplifier of structure as described above, for Raman-amplifying the signal light (which is the Raman amplifier according to the present invention).
In any of these Raman amplification pumping module, Raman amplifier, and optical communication system, the multiplexer multiplexes lightwaves with center wavelengths different from each other, which are emitted from the plurality of light sources incorporated in the pumping light generating system, to yield the pumping light and the pumping light is supplied into the optical waveguide such as an optical fiber or the like. The signal light propagating in the optical waveguide is then Raman-amplified in the optical waveguide. Particularly, in the present invention, the difference between center wavelengths of lightwaves emitted from a pair of light sources (with the center wavelengths of their output lightwaves adjacent to each other) out of the lightwaves emitted from the plurality of light sources is set less than 6 nm. If one should suffer a breakdown out of the two light sources with the difference below 6 nm between the center wavelengths as in this configuration, the output power of the other light source would be increased, whereby it is feasible to control the variation of the Raman amplification gain of the signal light at a low level in the Raman amplification optical fiber and, furthermore, to effectively suppress the degradation of the quality of the signal light in the optical communication system.
Specifically, where the lightwaves emitted from the plurality of light sources are subjected to polarization multiplexing in the Raman amplification pumping module according to the present invention, the pumping light generating system outputs 2N (N is an integer not less than 1) lightwaves with their respective center wavelengths different from each other and a difference is controlled below 6 nm between center wavelengths of the (2nxe2x88x921)th and the (2n)th (n is an arbitrary integer not less than 1 nor more than N) lightwaves out of the 2N lightwaves. In this case, if either one of the 2N light sources should suffer a breakdown, the output power of another light source having the center wavelength difference of output light below 6 nm from that of the broken light source would be increased, whereby the variation of the Raman amplification gain of the signal light can be controlled at a low level in the Raman amplifier. This effectively suppresses the degradation of the quality of the signal light in the optical communication system. The center wavelength spacing between a pair of light sources is practically 1 nm or more, preferably 2 nm or more in the case where the lightwaves emitted from the respective light sources are not subjected to polarization multiplexing.
In the Raman amplification pumping module according to the present invention, the pumping light generating system may comprise a configuration for effecting polarization multiplexing of lightwaves out of those emitted from the plurality of light sources and feeding the multiplexed lightwaves to the multiplexer. In this case, it is feasible to increase the power of the Raman amplification pumping light.
Further, the Raman amplification pumping module according to the present invention may comprise a depolarizer for depolarizing a polarization state of the pumping light outputted from the pumping light generating system. In the Raman amplification pumping module according to the present invention, the multiplexer may be configured to multiplex two lightwaves with the difference less than 6 nm between their respective center wavelengths in mutually orthogonal polarization states. In any of these cases, stable Raman amplification operation can be achieved.